Irritable bowel syndrome (IBS) which affects 8-14% of adults and accounts for 40-70% of visits to gastroenterologists, is a poorly understood disorder for which treatment is often ineffective. Objectives of the proposed research are to (1) resolve controversies about the pathophysiology of IBS by applying all the available techniques for myoelectric and motility measurement to a large group of IBS patients who fit strict diagnostic criteria and to appropriate controls, (2) determine the relationship among bowel physiology, alterations is stool frequency, and subjective bowel symptoms; and 3) investigate the psychological characteristics of patients with IBS which distinguish them from patients with similar bowel symptoms due to lactose intolerance. A research team consisting of psychologists and gastroenterologists will study 40 patients with IBS, 20 with lactose intolerance, 20 with diverticular disease, (DD), 20 with primary affective disorder (depression), 20 with generalized anxiety disorder, and 20 healthy subjects. The lactose intolerant group controls for the possibility that psychological symptoms in IBS are secondary to bowel symptoms since lactose deficiency produces similar bowel symptoms. The diverticular disease group are included to examine the specificity of the myoelectric and motility disorders associated with IBS since DD is the second most common motility disorder of the distal colon. The patients with primary affective disorder and generalized anxiety disorder (as defined in DSM-III) but without bowel symptoms control for the possibility that IBS is a psychological disorder involving misperceptions of normal bowel physiology and has no physiological basis. All subjects will give a clinical history of digestive symptoms and will take the Minnesota Multiphasic Personality Inventory and the Neuroticism-Extraversion-Opennese Inventory. Physilogical assessment will involve measurement of myoelectric and contractile activity 10 cm. and 25 cm. from the anal verge during baseline and following provocative stimulation with food, balloon distension of the colon, and cold stress. All participants will monitor and report bowel symptoms daily and will collect photographs of bowel movements for laboratory examination.